(1) Field of the Invention
The present invention relates to an image transfer device for use in an image forming apparatus, particularly relating to an image transfer device in which an image formed on the recording medium is electrostatically transferred to a sheet of paper.
(2) Description of the Prior Art
In the image forming apparatus using electrophotography, a photoreceptor which is made up of a conductive substrate (drum) coated with a photoconductive layer as a recording medium, is exposed to an optical image so that an electrostatic latent image is formed in accordance with this image. In order to visualize the latent image, toner as a coloring pigment is adhered thereto. The thus developed toner image is then electrostatically transferred to a transfer sheet which is fed in synchronism. Subsequently, the sheet is passed through the fusing fixing unit and discharged from the image forming apparatus body.
As a device for transferring the toner image formed on the photoreceptor, corona-chargers have been widely used, which supply opposite polarity to that of the static charge of the toner, to the undersurface of the paper sheet being conveyed, so that the toner image is electrostatically transferred to the paper sheet. Alternatively, in place of the corona charger, a device having a transfer roller which presses the paper sheet against the photoreceptor can be used. In this contact stage, the transfer roller is applied with a voltage opposite to the polarity of the toner, so that the toner is electrostatically transferred onto the paper sheet.
In stead of transferring the toner image formed on the photoreceptor as stated above, in some cases, the static latent image may be transferred by bringing a paper sheet, whose surface is treated with an insulator such as a dielectric layer etc., into intimate contact with the photoreceptor using a transfer roller etc. By this configuration, there is no longer a need to clean the photoreceptor of leftover toner after the toner image has thereon been transferred.
In contrast to those using the conventional corona charger or the transfer roller, it has been disclosed in Japanese Patent Application Laid-Open Hei 3 No. 231,273 that a transfer belt should be provided in order to convey the sheet which has had toner transferred, at the same time as transferring the toner image thereto. This transfer belt is tensioned between a pair of rollers; one of the rollers is abutted against the photoreceptor with the transfer belt inbetween and is supplied with a voltage for the transfer operation. FIG. 1 shows a typical example of it.
As shown in FIG. 1, a paper sheet P is fed inbetween a photoreceptor 10 and a transfer belt 11. One roller 12 which stretches the transfer belt 11 is applied with a voltage from a power source 14 so that an electric field is generated. This electric field causes the toner image formed on the photoreceptor 10 to electrostatically transfer to the paper sheet P side. The above transfer belt 11 is driven in synchronism with the rotation of the photoreceptor, by means of the driving roller 12.
In this way, in accordance with the transfer device shown in FIG. 1, roller 12, or one of the rollers for driving the transfer belt 11, is applied with a voltage for the transfer operation so that the toner image is transferred to the paper sheet P. Therefore, the paper sheet P after the toner has been transferred thereto is conveyed along the surface of the driven belt 11, to the next stage where the fixing operation is to be made. That is, the transfer device can also be used as the conveying device thus making it possible to make the apparatus compact.
Nevertheless, since in the transfer position of transfer belt 11, the photoreceptor 10 and roller 12 contact with one another in their arc portions, the contact range is narrow. That is, the contact range between the photoreceptor 10 and the transfer belt 11, which will form the transfer area, is extremely narrow so that the contact portion between the paper sheet P and the photoreceptor 10 becomes small. Hence, if the speed of the image formed in the image forming apparatus is raised, the passage time of the paper through the transfer position is shortened due to the shortness of the contacting time. This means that the necessary amount of charge cannot be imparted to the paper sheet to achieve an adequate transfer operation, thus causing transfer failure etc.
In order to eliminate the above problem, if the voltage to be applied to the roller 12 is increased, the transfer efficiency may be improved so that the transfer defect can be inhibited. The application of the high voltage, however, causes a large variation in the amount of charge to be imparted to the paper sheet P due to resistance fluctuation of the transfer belt 11. This variation directly affects as the dispersion of the transfer efficiency. This means that, when the toner image of `solid black` is transferred to the paper sheet, a pattern with various uneven spotted densities is transferred as it is, resulting in failures to produce a transferred image with even density. Thus, a measure against transfer failures could, on the contrary, promote the transfer failures.
The above problem of transfer failures can be eliminated if the resistance value of the transfer belt is set uniformly throughout the entire area thereof. However, it is very difficult to make the resistance value of the transfer belt uniform, and if it is done in manufacturing, the cost can become very high.